A communication system can be seen as a facility that enables communication sessions between two or more entities such as fixed or mobile communication devices, base stations, servers and/or other communication nodes. A communication system and compatible communicating entities typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. For example, the standards, specifications and related protocols can define the manner how communication devices can access the communication system and how various aspects of communication shall be implemented between communicating devices. A communication can be carried on wired or wireless carriers. In a wireless communication system at least a part of the communication between at least two stations occurs over a wireless fink.
Examples of wireless systems include public land mobile networks (PLMN) such as cellular networks, satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). A wireless system can be divided into cells, and hence these are often referred to as cellular systems. A cell is provided by a base station. Cells can have different shapes and sizes. A cell can also be divided into sectors. Regardless of the shape and size of the cell providing access for a user, and whether the access is provided via a sector of a cell or a cell, such area can be called radio service area or access area. Neighbouring radio service areas typically overlap, and thus a communication in an area can listen to more than one base station.
A user can access the communication system by means of an appropriate communication device. A communication device of a user is often referred to as user equipment (UE) or terminal. A communication device is provided with an appropriate signal receiving and transmitting arrangement for enabling communications with other parties. Typically a communication device is used for enabling receiving and transmission of communications such as speech and data. In wireless systems a communication device provides a transceiver station that can communicate with another communication device such as e.g. a base station of an access network and/or another user equipment. The communication device may access a carrier provided by a station, for example a base station, and transmit and/or receive communications on the carrier.
An example of communication systems attempting to satisfy the increased demands for capacity is an architecture that is being standardized by the 3rd Generation Partnership Project (3GPP). This system is often referred to as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. The LTE aims to achieve various improvements, for example reduced latency, higher user data rates, improved system capacity and coverage, reduced cost for the operator and so on. A further development of the LTE is often referred to as LTE-Advanced. The various development stages of the 3GPP LTE specifications are referred to as releases.
One aspect of LTE-Advanced Release 9 is that dual layer beamforming can be provided with a plurality of antennas which have a low correlation between different polarizations. Under typical radio channel conditions there can be at least two strong multiple input and multiple output (MIMO) sub channels with the plurality of antennas. In this way, a dual stream transmission can be achieved and improve cell throughput compared to a single layer beamforming transmission. The dual layer beamforming in 3GPP LTE release 9 can combine user equipment (UE) specific beamforming and spatial multiplexing. UE specific beamforming can be useful for time division duplex (TDD) systems as this arrangement can provide uplink and downlink channel reciprocity, which can be used to obtain instantaneous radio channel state information in order to generate near optimal precoding.
The radio channel state information can be obtain through measurements of channel state reference signals, such as sounding reference signals (SRS), which can be transmitted from the user equipment. The measurements of the channel state reference signals can be made such that each individual path between a transmit antenna and a receiver antenna can be measured.
In LTE there are agreements to leave the decision of supporting transmission of sounding reference signals (SRS) from all transmitting antennas up to the discretion of the user equipment manufacturers. In practice this means that a user equipment with, for example, two antennas can receive with two antennas but can only transmit with one antenna. This means that there may be no possibility for a base station to measure the full downlink channel state based on uplink channel transmission of sounding reference signals from a user equipment. This means that there may be only partial channel state information and this can impact the performance of dual-layer beamforming transmissions.
One proposal is to use dual layer beamforming with information based on the partial channel state information. R1-093515 3GPP TSG RAN WG1 #58 and R1-092631 3GPP TSG RAN WG1 #57bis suggest providing channel state information for the second layer based on second weighting vector mapping of a second largest eigenvalue (of channel state information). This can be obtained using a frequency granularity such as six physical resource blocks with a single antenna transmitting sounding reference signals. This only considers orthogonality between a first layer beamformer and a second layer beamformer and can lead to sub-optimal beamformers being used.
Other suggestions in Ozdemir et al “Opportunistic Beamforming with Partial Channel State Information” ICC June 2006 pp 5313-5318, Ozdemir et al “Opportunistic Beamforming over Rayleigh Channels with Partial Side Information” IEEE Trans. On Wireless Communication vol. 7 no. 9 pp 3416-3427 September 2008, and Toufik et al “MIMO OFDMA Opportunistic Beam forming with Partial Channel State Information” ICC 2006 pp 5389-5394 provide different arrangements of scheduling or feedback mechanisms for addressing partial channel state information.
It is noted that the above discussed issues are not limited to any particular communication environment, but may occur in any appropriate communication system where muting of data transmissions may be provided.